In this study we used immunocytochemistry (ICC) to localize expression of matrilysin, a matrix metalloproteinase (MMP) in the endometrium of spayed, hormone treated monkeys, and in monkey endometrial autografts. We have previously described the monkey endometrial autograft model, which facilitates sampling of monkey endometrium. In this model, spayed monkeys are treated for 14 days with estradiol (E2), and then E 2+ progesterone (P) for 8 days. The endometrial basalis is then collected by endometriectomy and transplanted subcutaneously. P treatment is continued for 1 month, then P is withdrawn and the grafts menstruate. Replacement and removal of the P implants, at 2 week intervals while the E implant remains in place creates artificial menstrual cycles. In this study, endometrial grafts were removed on each of days 0, 1, 2, 3, 4, 5 and 10 after P withdrawal (luteal-follicular transition). For comparison, the uterus was also collected from monkeys at these time points. Samples of monkey endometrium and the endometrial grafts were fixed and embedded in glycolmethacrylate for histological study, or were frozen and processed by ICC for localization of matrilysin. Both the grafts and the endometrium menstruated beginning on day 3 after P withdrawal, and repair of the luminal epithelium was underway by day 5. Matrilysin was detected by ICC in the endometrium in situ, by day 3. Staining intensity was greatest in the upper functionalis, decreasing in mid functionalis and absent in the basalis. Matrilysin staining was also observed in the mensing autografts where matrilysin was clearly induced in the glands and its staining intensity tended to be weakest in the deeper glands, similar to the intact uterus. This observation indicates that matrilysin plays an important role during menstruation, as it was upregulated similarly in both ectopic and eutopic endometrium during menses.